1. Field of the Invention
This invention relates to the modification and manipulation of the triboelectric properties of filter material to provide fabrics having predictable triboelectric properties for use as filter media. More particularly, the invention relates to the modification or adjustment of filter fabric media according to predictably calibrated triboelectric properties for use in dust collection operations so as to optimize the performance of the filters in relation to the particulate matter to be filtered.
2. Description of the Prior Art
The U.S. Environmental Protection Agency (EPA), in 1970, originally set forth a National Ambient Quality standard for particulate matter. Since that time, U.S. industry has commenced practices which reduced the mass of particulates on an average by 20% by the early 1980's, despite increased industrial developments. In view of further industrial expansion and especially in view of the projected greater use of coal to generate electrical power, particulate pollutants will have a tendency to increase unless appropriate control measures are taken. As regulations became more stringent, especially in regard to particle size, control devices for removing particulate matter became more restricted. The most common methods in increasing order of apparent potential acceptability based on performance are: mechanical collectors, particulate wet scrubbers, electrostatic precipitators, and fabric filters. Fabric filtration is a process not greatly different in principle of operation from that of the common home vacuum cleaner. Particulate matter is removed from a dirty air (gas) stream by virtue of separation processes that occur at or near the fabric surface. Five such mechanisms are identified in the separation process: inertial deposition, brownian movement, direct interception, gravitational sedimentation and electrostatics. Except for the electrostatic involvement feature, each of these has been well described in the prior art. Because filtration is among the most reliable, efficient and economical methods for removing particulate matter from gases, baghouses are being applied more universally for controlling emissions. As an example, baghouses may be applied to coal-fired utility boilers, and are one of the few air pollution control techniques easily capable of meeting the more stringent anticipated emission standards.
Although fabric filters are well known as being capable of collecting very small particulates, a high level of removal from industrial process gases is not routinely achieved. One reason for this is that not all fibers used in constructing the filter perform in the same manner, even where the chemical composition of the fibers is presumably identical in a favorably constructed fabric. Additionally, it appears that natural electrical forces clearly influence the filtration process. In fact, it appears that substantially all industrial processes produce particulate matter with charges, positive and negative. Although considerable information on the mechanics of the filtration process for uncharged particles is available, very little has been provided with regard to natural electrical effects in fabric filtration. This is despite the fact that particulate matter reaching the fabric filter is rarely uncharged and the medium itself is rarely devoid of an electric field. Accordingly, particles entering conventional collectors are mostly charged, sometimes far more extensively than at other times, but usually of mixed polarity. The type of generating process determines the magnitude of the charge, with grinding and other energy intensive operations producing particulate matter with extremely high levels of charge.
It is generally accepted that electrostatic attraction draws particles from the gas stream to fibers if the two are oppositely charged. Even if only one of the particles or filtering fabric is charged, a naturally induced charge will be created on the other. This results in a polarizing force that causes attraction and particle movement from the gas stream to the oppositely charged fiber. However, although as stated above, the particulate matter and/or the fabric filter may have electrostatic charges thereon, the polarity, magnitude and durability of the triboelectrically induced charge depends upon the inherent properties of the materials, including their chemical make-up and the electrical resistivity.
Electrical augmentation, the practice of electrically charging the gas-entrained particles and/or applying an electric field to the collecting medium, can provide excellent filtration features. These artificial charging conditions are, however, applicable only to non-combustible, electrically chargeable particles. Another limitation is that they require special processing and collection facilities, electrodes, electrical circuits, and the like. The most commonly proposed electrical augmentation techniques utilize a corona discharge to impress a charge on the particulate matter and/or a high D.C. voltage on wire electrodes appropriately located on or near the surface of the collecting fabric. One of the more serious limitations of electrode systems proposed for such augmentation is the short life of the circuitry.
A very significant portion of the improved filtration performance gained by electrical augmentation or artificial charging may be achievable simply by balancing the natural charging properties of the fabric with those provided by the particulate matter. By utilizing natural charges, that is by using a fabric filter medium of appropriate inherent triboelectric properties relative to those of the particulate matter being collected, it is possible to deposit a low air-flow resistant cake without electrical augmentation. By suitably balancing the natural triboelectric properties of the medium in relation to those of the particles being collected, conditions are realized for approaching the ultimate level of filtration performance now attained only by electrical augmentation.
Practically all of the commercial fibers used for filtration fabrics respond to contact electrification and because of molecular variations, gain or lose electrons differently. Different fibers, therefore, become charged at different polarities. When listed in a downward order from electropositive to electronegative, a series may be developed, referred to as the triboelectric series (TE), and any material fabric or dust, may be included according to its electrostatic polarity relative to others in the list. Triboelectrification is the frictional process by which substances such as fabrics, particles, and the like, when abraded or rubbed by other substances and separated, develop electrostatic charges. Polarity of the acquired charge on the rubbed material to that on the rubbing substance depends upon the inherent character of the rubbed substance. The magnitude of the acquired charge depends upon various qualities of both the rubbed and the rubbing materials including the differences in their spacing in the triboelectric series, the roughness of their surfaces, the environment to which they are exposed and other parameters. Natural charging refers to the charging process that occurs naturally in the course of handling materials of all types. Particulate matter acquires electrostatic charges by contact with or rubbing against other substances such as the walls of ducting or during formation/production as generated at high temperatures, grinding, and the like. The triboelectric properties of fabrics generally are either not significant or have not been recognized to be critical or useful in their normal service applications.
In dry filtration, however, this characteristic of the medium appears to control the process and dictate its performance. An ideal balance between the electrostatic charges on the collecting filter medium provides optimal or near optimal filtration parameters in terms of pressure drop, efficiency, gas flow-through, fabric cleanability and other dependent variables. Although non-electrically augmented filtration operations presently are anticipated to be the most common collection methods, controlling the filter operation by balancing the electrostatic properties of the particles and of the filter medium has received little or no consideration.
The opportunity to utilize natural electrostatic effects fully has been restricted to some extent by non-availability of appropriate media and most seriously by the triboelectric limitations of commercially available fabrics. The most serious problems have included triboelectric non-uniformity among even supposedly identical fabrics, and the limitations of the inherent triboelectric properties of an otherwise suitable fabric. The fabrics marketed for filter media use presently do not always permit the choice of the desired triboelectric properties, neither are the fabrics constructed from blends of fibers having a selectable preferred balance of electropositive and electronegative fibers for filtering gas entrained particles of both charges, to optimize the process.